[feat] react-x/jsx-uses-vars for MDX and other non-TS envs

[karlhorky]

Describe the problem

Similar to #846

Originally reported in #85 (comment)

I ran into an issue today that eslint-plugin-mdx requires react/jsx-uses-vars

There is not yet full TypeScript support for MDX files, and it appears ESLint doesn't support multiple parsers for a set of files (eg. for trying to use typescript-eslint parser along with the MDX parser).

I tried to use eslint-plugin-react-x for this (looking for react-x/jsx-uses-vars) and found this part of the #85 issue description:

The following can be enforced by TypeScript, no need to implement them

• react/jsx-uses-vars

Describe the solution you'd like

What do you think about implementing react-x/jsx-uses-vars for other languages like MDX? (and also just for completeness, for other situations where TypeScript cannot be used)

Alternatives considered

Keep using react/jsx-uses-vars via eslint-plugin-react

Additional context

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